1. Field of the Invention
The present invention relates to a ferrite magnetic material to be suitably used for a sintered magnet, a magnet powder and the like, in particular, a ferrite magnetic material capable of attaining non-conventionally high magnetic properties even by sintering at a relatively low sintering temperature.
2. Description of the Related Art
For example, as ferrite magnetic materials to be used for sintered magnets, hexagonal Ba ferrites or hexagonal Sr ferrites are known, and at present, the magnetoplumbite-type (M-type) Ba ferrites or the M-type Sr ferrites are predominantly used. The M-type ferrite is represented by a general formula AFe12O19, in which Ba or Sr is applied to the atom constituting the A-site. An M-type ferrite in which Sr is adopted for the element constituting the A-site, a part thereof is substituted with a rare earth element, and further a part of Fe is substituted with Co is known to have high magnetic properties (residual magnetic flux density and coercive force) (Patent Documents 1 and 2). As has been established, it is essential that the M-type ferrite contains La as the rare earth element. This is because La is highest among the rare earth elements with respect to the solid solubility limit amount in the M-type hexagonal ferrite. Patent Documents 1 and 2 disclose that the use of La as the substitution element for the A-site constituting element permits the increase of the amount of Co solid solution to substitute a part of Fe and contributes to the improvement of the magnetic properties.
When the A sites in an M-type ferrite magnetic material are occupied exclusively by Ca, no hexagonal ferrite can be formed, and accordingly such an M-type ferrite magnetic material has not been used as a magnet material; however, as is known, even when Ca is an element constituting the A site, the addition of La permits the formation of a hexagonal ferrite, and the further addition of Co results in manifestation of high magnetic properties (Patent Document 3). Such a ferrite magnetic material can be described as an M-type ferrite in which Ca is selected as the element constituting the A site, a part thereof is substituted with at least one rare earth element (surely comprising La), and further a part of Fe is substituted with Co.
Additionally, Patent Document 4 discloses a ferrite sintered magnet comprising Ca as an element constituting the A site. Patent Document 4 aims at the retention of a high coercive force not to be degraded even when the ferrite sintered magnet is formed to be thin while maintaining a high residual magnetic flux density, the composition of such a ferrite sintered magnet being specified by the following general formula:A1−x−y+aCax+yRy+cFe2n−zCoz+dO19 (in atomic ratios)
In this formula, the element A includes Sr, or Sr and Ba; the element R is at least one of the rare earth elements inclusive of Y with the proviso that the element R surely includes La; x, y, z and n represent the contents of Ca, the element R and Co, and the molar ratio, in a calcined body, respectively; a, b, c and d represent the contents of the element A, Ca, the element R and Co, added in a pulverizing step of the calcined body, respectively; and these quantities respectively satisfy the following conditions:0.03≦x≦0.4, 0.1≦y≦0.6, 0≦z≦0.4, 4≦n≦10, x+y<1, 0.03≦x+b≦0.4, 0.1≦y+c≦0.6, 0.1≦z+d≦0.4,0.50≦[(1−x−y+a)/(1−y+a+b)]≦0.97,1.1≦(y+c)/(z+d)≦1.8, 1.0≦(y+c)/x≦20, and0.1≦x/(z+d)≦1.2.
Further, Patent Document 5 discloses a ferrite sintered magnet containing Ca as an element constituting the A site. The ferrite sintered magnet of Patent Document 5 has a composition represented by a formula (1−x)CaO.(x/2)R2O3.(n−y/2).Fe2O3.yMO, in which R includes at least one element selected from La, Nd and Pr with the proviso that R surely includes La, M includes at least one element selected from Co, Zn, Ni and Mn with the proviso that M surely includes Co, and x, y and n representing molar ratios respectively satisfy the relations 0.4≦x≦0.6, 0.2≦y≦0.35, 4≦n≦6, and 1.4≦x/y≦2.5.
Patent Document 1: Japanese Patent Laid-Open No. 11-154604
Patent Document 2: Japanese Patent Laid-Open No. 2000-195715
Patent Document 3: Japanese Patent Laid-Open No. 2000-223307
Patent Document 4: International Publication No. 2005/027153
Patent Document 5: Japanese Patent Laid-Open No. 2006-104050